This invention relates to a filled type ink-jet recording sheet. More particularly, it relates to an ink-jet recording sheet for use in an ink jet recording system that employs a water-based ink, the sheet exhibiting a high rate of absorption of ink deposited on the recording sheet so as to allow a clear color to develop, providing an image having circular dots and sharply defined edges, having a high surface strength and the same touch and feel during use as plain paper, and being able to record images with high resolution at high speeds.
In order for an ink-jet recording system to be able to produce high quality recorded images, it is required that the substrate employed be such that the ink deposited on the surface of the recording sheet which is composed mainly of paper spreads to form a true circle and is absorbed quickly in a controlled manner without becoming irregularly spread on the surface, and that the surface structure be such as to promote coloration of the deposited ink. Various proposals have so far been put forward with a view to realizing these requirements.
For example, it has been reported that when a pigment such as silica is coated on one surface of paper together with a binder, the whiteness and clarity of silica are exploited to provide better color formation and ink absorption than in the case of plain paper, and in another report, the use of high-size paper, and in still another report lowsize (Stockigt sizing degree of no more than 1 second) or non-sized paper as base paper for coating was proposed (see Japanese Patent Publication No. 27588/1985).
However, coated type ink-jet recording paper has a peculiar surface on the coated layer which is mainly composed of silica and the loss of the natural feel of the paper makes it unpopular in printing and business form applications. The coated type ink-jet recording paper also has problems in use such as low surface strength and threading speed. Therefore, uncoated fine paper has conventionally been used in the above-mentioned applications.
It is known that uncoated ink-jet recording paper can be produced with a liquid-absorbing pigment such as silica being incorporated in the process of papermaking or without adding any sizing agent. However, when ink is deposited on non-sized ink-jet recording paper having a liquid-absorbing pigment incorporated therein, it is extensively absorbed by the paper layer to be blocked by pulp fibers and pigment particles, thereby producing an unsharp or less bright color in the layer where ink is absorbed. In addition, the spread and shape of ink dots are often irregular and lack sharpness, and the image obtained is not as sharp as what is produced from coated type paper that achieves selective ink absorption by virtue of the surface layer.
Substantially similar problems, namely, diffuse edges and irregular dot shapes, have occurred when a coated layer is formed on the non-sized paper shown in Japanese Patent Publication No. 27588/1985.
In the presence of a fairly large amount of a liquid-absorbing pigment, a process of sheet production utilizing a given papermaking method is not consistent and the resulting irregular distribution of pigment in the paper layer can cause unevenness or variations in the color of images or troubles in use such as the generation of paper dust.
The use of acidic paper or alkaline paper has also been reported but they involve the disadvantage that the dye of the ink might be decomposed or otherwise deteriorate by the action of acids or alkalis.
The above-mentioned problems with coated ink-jet printing paper and with filled type paper have chiefly originated because of the absence of thorough reviews concerning any influence on an ink jet image caused by the quality of the structure of a paper layer composed of wood pulp and pigments (which are the essential ingredients of ink-jet printing paper of either coated or filled type) and its physicochemical properties, in particular, the ink absorbing capability typified by sizing degree.